Spironolactone‐induced XPB degradation depends on CDK7 kinase and SCF^FBXL18 E3 ligase

Abstract

The multisubunit complex transcription factor IIH (TFIIH) has dual functions in transcriptional initiation and nucleotide excision repair (NER). TFIIH is comprised of two subcomplexes, the core subcomplex (seven subunits) including XPB and XPD helicases and the cyclin‐dependent kinase (CDK)‐activating kinase (CAK) subcomplex (three subunits) containing CDK7 kinase. Recently, it has been reported that spironolactone, an anti‐aldosterone drug, inhibits cellular NER by inducing proteasomal degradation of XPB and potentiates the cytotoxicity of platinum‐based drugs in cancer cells, suggesting possible drug repositioning. In this study, we have tried to uncover the mechanism underlying the chemical‐induced XPB destabilization. Based on siRNA library screening and subsequent analyses, we identified SCFFBXL18 E3 ligase consisting of Skp1, Cul1, F‐box protein FBXL18 and Rbx1 responsible for spironolactone‐induced XPB polyubiquitination and degradation. In addition, we showed that CDK7 kinase activity is required for this process. Finally, we found that the Ser90 residue of XPB is essential for the chemical‐induced destabilization. These results led us to propose a model that spironolactone may trigger the phosphorylation of XPB at Ser90 by CDK7, which promotes the recognition and polyubiquitination of XPB by SCFFBXL18 for proteasomal degradation.

Overcoming cancer therapeutic bottleneck by drug repurposing

“…This is due to spironolactone reversibly inducing the degradation of XPB in a ubiquitin-activating enzyme-and proteasome-dependent manner, impairing both basal and activated mRNA transcription of the transcription/repair factor TFIIH. 374,375 In agreement with this, a recent study also demonstrated that spironolactone could inhibit SIRT2-mediated transcription-coupled nucleotide excision repair, disturbing cisplatin-induced DNA crosslinks in lung cancer. 376 In addition, the function of spironolactone on reducing homology directed repair frequencies was confirmed.…”

Human T-Cell Lymphotropic Virus Type 1 Transactivator Tax Exploits the XPB Subunit of TFIIH during Viral Transcription

“…Moreover, to discriminate between endogenous and overexpressed XPB, an XPB-GFP construct was used and its impact compared to that of XPD-GFP. Importantly, fusion to green fluorescent protein (GFP) was previously shown to preserve incorporation into TFIIH and activities of both XPB and XPD (31,41). In addition, we found that XPB-GFP retained the ability to coprecipitate with Tax (Fig.…”

“…Strikingly, SP is believed to induce the degradation of XPB within preformed TFIIH complexes, removing XPB while preserving TFIIH integrity (21,30). A recent study showing that SP-induced XPB degradation depends on its prior phosphorylation by the other TFIIH subunit CDK7 provides the molecular explanation for this selectivity (31).…”

Cell-type specific concentration regulation of the basal transcription factor TFIIH in XPBy/y mice model

“…In fact, blocking transcription induces apoptosis in proliferative cells but does not spare non-malignant cells, which may suffer from this kind of treatments. Specifically, two drugs have been developed to target TFIIH: (i) triptolide that covalently binds to the XPB subunit of TFIIH and inhibits its ATPase activity [33], triggering CDK7-dependent degradation [24] and hence disrupting both RNAP2 and RNAP1 transcription [37]; (ii) spironolactone that induces XPB degradation [1, 34]. Nevertheless, these drugs have the same drawbacks as other drugs blocking transcription since their use may hinder the cellular activities of normal non-malignant cells.…”